18 week old chicken died... Why?

[jschway]

I had an 18 week old chicken die on me the other day. The rest of the flock seems to be doing great. (Its been about 3 weeks ago)
She did not leave the coop one day and was simply sitting on the coop floor. She was on her side stiff in a few hours...
What do you think?

Facts:
I switched to layer feed when they were 15 16 or 17 weeks old. Would this have cause an egg to be too large for her to pass?
It was hot but they had water and were free range. (Upper 90s to 100)

Thanks